Description
TypeScript Version: 2.4.0
Code
// The following works no problem
type ExhaustiveType = Array<number | undefined>;
const test1 = (value: ExhaustiveType) => value.filter(v => v === 2);
/** Cannot invoke an expression whose type lacks a call signature. Type '{ (callbackfn: (this: void, value: number | undefined, index: number, array: (number | undefined)...' has no compatible call signatures. */
type ExcessiveType = number[] | Array<number | undefined>;
const test2 = (value: ExcessiveType) => value.filter(v => v === 2);Expected behavior:
Both examples compile.
Actual behavior:
The latter example fails with Cannot invoke an expression whose type lacks a call signature. Type '{ (callbackfn: (this: void, value: number | undefined, index: number, array: (number | undefined)...' has no compatible call signatures.
Explanation
ExhaustiveType and ExcessiveType both represent exactly the same types. The latter is more explicit, and while in this particular example it might look silly, w/ more complex types and type aliases, being more explicit allows for more expressivity. IMHO TypeScript could at the time of defining ExcessiveType, 'reduce it down' to the ExhaustiveType. Alternatively (to preserve expressiveness e.g. in the IDE), the compiler could internally treat them as equivalent.